Ruling engines for the production of fine pitch scales



J- DYSON May 14, 1968 RULING ENGINES FOR THE PRODUCTION OF FINE PITCH SCALES Filed Nov. 16, 1964 United States Patent 3,382,761 RULING ENGINES FOR THE PRODUCTION OF FINE PITCH SCALES James Dyson, Teddington, England, assignor to National Research Development Corporation, London, England, a corporation of Great Britain Filed Nov. 16, 1964, Ser. No. 411,294 Claims priority, application Great Britain, Nov. 18, 1963, 45,507/ 63 11 Claims. (Cl. 88-14) ABSTRACT OF THE DISCLOSURE A ruling engine for diffraction gratings having a continuously traversed blank carriage and an interferometer for co-ordinating operation of the tool actuating mechanism with the carriage traverse avoids errors due to change in the air conditions by including variable length evacuated tubes in the interferometer beams controlled from the carriage traverse to maintain a constant length path of the beams through air. Preferably one tube is shortened and the other lengthened by equal amounts during the traverse, Preferably the correction is applied through a servo system to the tool actuating mechanism, Errors in the tool actuating mechanism may be corrected by a secondary interferometer and a secondary servo system. Particular constructions of servo mechanisms together with particular optical and photo electric devices for controlling the operation of the engine described.

The present invention relates to ruling engines for the production of fine pitch scales such as diffraction gratings.

It is known to arrange such a ruling engine so that the blank carriage moves continuously and it is also known to control the operation of the engine by means of a servo system which obtains its input information from an interferometer which measures the longitudinal traverse of the blank. Since it is hardly practicable to enclose the complete engine in a constant pressure enclosure, errors still arise due to variation in the refractive index of the air through which the interferometer beams pass resulting from such factors as variations in the pressure, temperature, humidity and carbon dioxide content of the air.

The weight of the blank carriage and the frictional resistance to its movement necessitate the use of a power driven screw or similar power driven mechanism to carry the main burden of moving the carriage and the function of the servo system is to introduce corrections in the movement produced by the power drive mechanism. Just because the carriage is very heavy if as in the past the corrections are applied to the carriage, the speed of response of the correcting system is limited and is liable to over-shoot and uncertainty.

Another source of error is the errors and deformation in the parts guiding the tool, usually a diamond, in its traverse so that its path over the blank departs from a fixed relationship to the stationary element (usually a mirror) of the interferometer.

To deal with variations in the refractive index of the air through which the interferometer beams pass, the invention provides for the total length of the passage through air to be the same for both beams. Then any variation in refractive index affects both beams equally and is without effect on the fringes.

The desired result can be obtained by providing a variable length evacuated tube in the path of that beam of the interferometer which changes with the displacement of the carriage and coupling this tube to the carriage.

It is desirable to keep the overall length of the interferometer beams at a minimum while it is not practical to 3,382,761 Patented May 14, 1968 construct a variable length evacuated tube having a range of change of length more than a little less than 2:1, and because it is also desirable to have the overall path difference between the two beams approximately zero at the mean position of the carriage it is preferred to provide a variable length evacuated tube in the path of each of the beams. Then so far as the length of the tube in the beam of changing length is changed by an amount less than the displacement of the carriage, to maintain the lengths of the paths of the two beams in air equal, the length of the other tube must be changed in the opposite sense by the difference between the change in the length of the former tube and the displacement of the carriage. Thus if the length of the former tube is changed by half the displacement of the carriage the length of the other tube will equally need to be changed by half the displacement of the carriage. This arrangement enables the overall length of the interferometer beams to be kept at a minimum while enabling the over-all path difference between the two beams to be made approximately. zero at the mean position of the carriage.

When two evacuated tubes are used in this way their lengths are conveniently changed by auxiliary screws geared to the mechanism which feeds the blank carriage. It can be shown that the accuracy of these screws and the gearing which drives them need only be that associated with good engineering practice.

To avoid the difficulties and shortcomings arising from the application by the servo system of the corrections to the blank carriage, the invention provides for these corrections to be applied through the diamond actuating mechanism. Thus the carriage is displaced continuously by the feed screw or the like and the corrections are applied to varying the phase relationship of the diamond actuating mechanism to the feed mechanism.

For this purpose the moving fringes derived from the interferometer as the blank carriage moves may fall on a photo-electric cell and directly generate an alternating current, or preferably indirectly generate an alternating current of higher frequency, which alternating current is used to operate a synchronous motor which operates the mechanism by which the diamond carriage is reciprocated.

To deal with errors and deformation in the parts guiding the diamond in its traverse across the blank, the pres ent invention provides another interferometer (which will hereinafter be referred to as the secondary interferometer, while the interferometer which controls the longitudinal feed of the blank will be referred to as the main interferometer) by which the position of the ruling diamond with respect to the stationary element, usually a mirror, of the main interferometer is measured and a secondary servo system controlled by the secondary interferometer by which any necessary correction is applied to the position of the tool.

The accompanying highly diagrammatic drawings illustrate an example of embodiment of the invention; FIGURE 1 being a plan view and FIGURE 2 a detail side elevation.

The diamond '11 is held in a shank 12 which in turn is held in an arm 13 which is pivoted to a member 14 in a. known manner by means of flexible strip hinges 15. The member 14 is mounted on the diamond carriage 16 reciprocable on fixed ways 17.

A small Wollaston prism 18, modified to obtain symmetry, is secured to the shank 12, as close to the diamond as possible. These parts are enclosed within an optical system consisting of two identical concave mirrors 19, 21 connected by two bars 22, 23 of low expansion material such as fused silica. To avoid fouling the grating blank 24 on the ruling carriage 25, only narrow diametral strips of mirror are used. The radii of the mirrors are such that the centre of curvature of each lies in the face of the other, and both mirrors are pierced by an axial aperture as shown. The line of centres of the mirrors is parallel to the direction of reciprocation of the diamond carriage 16, and the ways 17 and mirror system 19, 21 are rigidly fixed to the engine frame 26.

Light from a lamp 27 is polarized by a polarizer 28 and enters through the aperture of mirror 19. It is divided into two beams by the Wollaston prism 18 which, after successive reflections by the mirrors 19, 21, enter the prism again and are re-emitted, emerging through the aperture of the other mirror 21. The operation of this system is then as set out in British patent specification No. 773,238; a motion of the Wollaston prism in a direction perpendicular to the line of centres of the mirrors leads to a path-difference between the two coincident polarized beams which emerge through the aperture of the mirror 21.

These two beams pass through an analyzer 29, and a variation of the path-difference leads to a variation in intensity of the light emerging from the analyzer. This light enters a photo-cell 31. Before the light reaches the analyzer it traverses a phase modulator 32, which may consist of a crystal of ammonium dihydrogen phosphate bearing transparent electrodes 33 between which an AC. voltage is applied. This is known to introduce a further path-difference between the two polarized beams which is proportional to the voltage.

Assume that, when the diamond is moving along the required line of ruling, the path-difference (in the absence of any voltage on the modulator 32) is zero. The light entering the photo-cell 31 is then a maximum. If the A.C. voltage is then applied to modulator 32 the light will be modulated in intensity and so will the photo-current from cell 31. Under these conditions, however, the AC. component of this current will contain no component of fundamental frequency.

If the Wollaston prism now moves a little in a direction normal to the line of centres of the mirrors, a path-difference is introduced, and a component of fundamental frequency appears in the photo-current. If the path-difference is small as compared with a wavelength, this fundamental component will be a measure, in magnitude and sense, of the movement of the Wollaston prism. It can be detected by a phase-conscious rectifier 34 which delivers a DC current also corresponding to the movement of the Wollaston prism. This can be used in a known manner (e.g. by use of a piezo-electric element 35) to return the Wollaston prism, and hence the diamond, to its correct path, the parts being mounted to permit the minute lateral movements involved. For example the arm 13 may be slightly flexible in the required direction or hinged by a hinge free from any looseness. This is indicated in the drawing by a thinning of the arm 13 in the appropriate direction at 13a.

This path will be fixed with reference to the mirrors 19, 21. Hence the fixed mirror of the interferometer which measures the motion of the blank carriage may appropriately consist of a polished and silvered area 36 on the silica bar 22. This system ensures that the position of the mirror 36 is a true indication of the path of the ruling diamond.

In the main interferometer, a Kosters prism 37 divides an incoming beam into component beams 38, 39. Beam 38 is reflected from the mirror 36 and returns; beam 39 is reflected from a mirror 41 fixed to the blank carriage 25. The two beams traverse telescopic systems of tubes 42, 43 and 44, 45 which are sealed at the outer ends by windows 42a, 43a and 44a, 45a respectively. The sliding joint between the two tubes of each system is sealed by sliding rubber sealing rings 46, 47 respectively in a manner well known in the vacuum art, and the tubes are evacuated through tubulations 48, 49.

The blank carriage 25 is moved along ways 51 by a screw 52 which is rotated at a suitable and approximately constant speed by a worm wheel and worm 53 and the telescopic systems 42, 43 and 44, 45 are extended or retracted by respective screws 54a, 5411 which are coupled to screw 52, in such a way that the lengths of those parts beams 38 and 39 which lie in air are equal. Since the two telescopic tube systems 42, 43 and 44, 45 when closed must be at least substantially half as long as when fully extended, in order to keep the over-all length of the beams as short as possible, the tube systems should be arranged with the stationary windows 42a, 44a at as nearly as possible the same longitudinal position and so that in the position shown, with the carriage 25 in midposition each system is substantially half extended. Also the distance between mirror 36 and window 43a and that between mirror 41 and window 45a is made equal and in this position should be equal substantially to a quarter the total distance the carriage 25 can be moved; as shown these distances 36-4311 and 4145a are much greater but this is only to avoid undesirable complication of the drawing. The gearing and pitch of the screw 5417 are made such that the tube 45 moves in the same direction as but at only half. the rate of the carriage 25. Thus if from the position shown the carriage 25 moves to the right in FIGURE 1, it will overtake the window 45a and if the minimum distance is employed at the end of its travel the mirror 41 will have just reached the window 45a. Accordingly the passage of the beam 39 in air will have been reduced a quarter of the total path of the carriage. Since the mirror 36 is stationary, in this same period to keep the passage of the beam 38 in air equal to that of the beam 39, it will be necessary for the system 42, 43 to be extended at the same rate as the system 44, 45 has been closed. Consideration will show that when the carriage 25 moves to the left from the mid-position shown the requisite adjustments of the telescopic systems will occur. The arrangement described also provides the minimum overall length to the left of the windows 42a, 44a, while the Kosters prism 37 can be located as close to the Windows 42a, 44a as practical convenience will allow. In practice the telescopic tube systems cannot close down quite to half their extended length nor can the mirrors 36, 41 be allowed actually to touch the windows 43a, 45a and a corresponding small increase in path length must be allowed.

The drawing shows the mirror 36 longitudinally displaced somewhat With respect to the mirror 41 at midposition. This necessitates the system 42, 43 being correspondingly longer than the system 44, 45 and it also results in an overall path difference between the two beams at mid-position not being zero as is desirable, but equal to the longitudinal distance between the mirrors 36, 41. In practice however there is no real difliculty in mounting the mirror 41 to avoid this longitudinal displacement at mid-position and it is only shown in the diagrammatic drawing to avoid obscuring the drawing. Indeed there are other differences which would be made in practice. Thus the two telescopic systems have been shown to one side of the screw 52. They have also been shown side by side in the horizontal plane. This has been done solely for the sake of clarity. In practice to avoid errors due to small rotation of the blank caused by lack of straightness of the ways 51, the telescopic systems would be arranged in the vertical plane through the screw 52. They should also be arranged as close together as possible.

The diamond carriage 16 is reciprocated by means of a connecting rod 55 which is actuated by a mechanism 56, which may be one of many known forms, which translates steady rotational motion into a reciprocating motion which, over at least a major portion of one stroke, is of constant velocity. The hinge 15 enables the diamond to be lifted clear of the blank at other times by parts of the known mechanism not shown.

The dividing layer 57 of the Kosters prism 37 is made in the form of a polarizing interference filter of known form which reflects light polarised normal to the plane of incidence and transmits light polarised in that plane. A lamp 58, which may desirably be a gas optical maser, delivers monochromatic light to the prism. Rays 59 and 61 are typical rays, each representing one half of the total beam. The light is polarised at 45 with the plane of incidence by a polarizer 62. A quarter-wave plate 63 suitably oriented, acting as a half-Wave plate by reason of the double passage, interchanges the polarization of the two beams to facilitate their second passage through the prism.

After emerging from the prism the two rays traverse a phase modulator 64 similar to 32, and an analyzer 65, then enter photo-cells 66 and 67. Ray 61, however, traverses a quarter-wave plate 68 before the analyzer.

As the carriage 25 proceeds, the intensity of light at the photo-cells 66, 67 fluctuates sinusoidally with carriage motion.

If, however, an AC. voltage of frequency f is applied to the modulator 64, and only the fundamental component of the light variation is considered, it can be seen that the current from, say photo-cell 66 will have an AC. component of frequency f which is modulated by a sinusoidal envelope corresponding to the carriage motion. The light falling on photo-cell 67, having passed through the quarter-wave plate 68 and so having suffered a further path-difference of a quarter of a wavelength be tween its two polarized components, will give rise to a current from photo-cell 67 of which the AC. component is modulated by a sinusoidal envelope which is in quadrature to that from photo-cell 66.

Thus, if the carriage has moved a distancex from that giving zero path-difference in the interferometer, the envelopes of the curents from cells 66 and 67 respectively will be proportional to sin 41rx/)\ and cos 41rx/ where A is the wave-length of the light.

These currents are fed (after, if necessary, conversion by a well-known method to a three-phase system) to a first synchronous motor constituted by a magslip 69, the rotor of which then, in the absence of mechanical load, rotates through an angle which is equal to 21rx/)\. If this could be used to drive the mechanism 56 directly, the problems would be solved; however, the required torque would be such that the angle of rotation would depart seriously from the value 21rx/ To avoid this, the currents might be amplified and operate a more powerful synchronous motor but it is preferred to use the magslip 69 to actuate a generator which not only imposes a very small torque load on the magslip but also serves to generate an alternating current at a higher frequency with advantages pointed out later, which operates a second synchronous motor which operates the diamond-reciprocating mechanism. In the illustrated example the motor of the magslip 69 drives only a disc 71 mounted on its shaft. On the periphery of this disc has been recorded, by standard tape-recording techniques, a sinusoidally varying magnetic pattern with n complete sinusoids in the circumference. The disc may be of steel to receive the recording or may be of composite construction. Three pick-up heads 72, 73 and 74 are so disposed that the voltages picked up from the disc as it rotates form a balanced 3-phase system. These are amplified and passed to a 3-phase synchronous motor 75 which drives the mechanism 56. The motor 75 then runs at n times the speed of the magslip 69 and so needs to exert only l/n of the torque required to drive the mechanism 56 directly. Also, a lag in the angular position of the rotor of motor 75, due to the mechanical load, gives rise to only 1/11 of the error that a similar lag of the rotor of magslip 69 would cause.

As an example, if fringes pass for each line ruled and 12 lines are ruled per minute, the magslip 69 rotates at 60 rpm. If a maximum error of ,1 fringe is allowed in line position, the angle through which the rotor of motor 75 turns must depart from the nominal value :by

only of the angle corresponding to one complete cycle of the ruling engine. In the case envisaged, this latter would amount to 5n turns. If 12:50, say, the motor 75 runs at 3,000 rpm. and makes 250 revolutions in one machine cycle. The allowable error is then revolution, which the motor will achieve if it is to maintain synchronisation at all.

By this system, the motions of the carriage and the ruling tool 11 are locked together as if a perfect screw were used rather than by a servo system; thus, troubles due to stability do not arise. The speed of correction of an error is limited by the inertia of the magslip 69 and motor 75 which are vastly less than that of the carriage 25.

If a sudden jump in the motion of the carriage occurs, due to a sudden release of elastic energy, the magslip 69 will follow this if it does not exceed half a fringe. It should not be greater than this in a suitably designed engine. The motor 75 might not respond fast enough, however, and might lose synchronism momentarily; having fallen out of step by a whole number of cycles of its driving supply, it will have no tendency to recover. Hence a back-up system is necessary to avoid this, that is to say a system such that departures from synchronism of the motor 75 amounting to a whole number of cycles of its supply are detected by comparing a periodic signal derived directly from the carriage feed mechanism with a signal derived from the main interferometer beam, and the resulting difference signal through a servometer imposes a corresponding correction through one member of a differential gear 88 by which a motor 87 is coupled to the mechanism 56. In the illustrated example a beam 76 is taken from the interferometer on emergence from the Kosters prism 37 (the nature of the means 77 for doing this are not shown, for clarity, but means suitable for the purpose are well known, e.g. a glass plate which reflects a small fraction of the light) and passes through two quarter-wave plates 78, 79, between which is a rotatable half-wave plate 81 mounted in a ring which is driven by gearing 82 from the ruling mechanism at such a speed that, for every fringe which passes in the interferometer, the plate 81 rotates through a quarter of a turn if the system is working properly.

It is known that the system comprising 78, 79 and 81 introduces a path-difference between the two polarized components of the beam 76 which is proportional to the angle of rotation of plate 81 and equal to four wavelengths for each rotation thereof. Hence, if the sense of rotation is properly chosen, the path-difference between the components of the emerging beam will not vary as the carriage advances.

If, however, the motor 75 slips a number of cycles as described, the path-difference of the emerging beams increases. This is detected by a phase modulator 83, an analyzer 84 and a photo-cell 85, the output of which is fed into a phase conscious rectifier 86. In the manner previously described, the output of the rectifier indicates in sense and magnitude the departure of the path-difference from zero. This output is fed into a servo-motor 87 which controls the motion of the cage of the so-called differential gear 88 interposed between the motor 75 and the mechanism 56, and so introduces a correcting rotation into the mechanical train. This correcting rotation is limited, by known mechanical means (such as, e.g., a Maltese cross mechanism) to that corresponding to a whole number of rotations of the motor 75. When the resulting error has been reduced to less than one rotation of the motor 75 the latter will pull into synchronisation and complete the correction itself.

1. A ruling engine for the production of fine pitch scales comprising a blank carriage, means for continuously traversing said carriage longitudinally, a main interferometer incorporating an optical element moved by said carriage whereby a first light beam is caused to vary in length in a direct relationship to the traverse of said carriage and to interfere with a second light beam of fixed length to provide an output which is a measure of the traverse of said carriage, a tool and actuating mechanism therefor for making scale markings on a blank carried by said carriage, a servo system to which the output of said main interferometer is applied and by which said tool actuating mechanism and the traverse of said carriage are co-ordinated, at least one variable length evacuated tube which is included in the path of said first light beam, and means coupling said tube to said carriage whereby the length of said tube is changed in such a way that the total length of the passage through air is the same for both said beams.

2. A ruling engine according to claim 1 also comprising a second variable length evacuated tube which is included in the path of the second of said beams, and means coupling said second tube to said carriage, said means coupling both said tubes to said carriage changing the length of said one tube by an amount less than the displacement of the carriage, and changing the lengih of said second tube in the opposite sense by the difference between the change in the length of said one tube and the displacement of the carriage.

3. A ruling engine according to claim 2 in which said means coupling said tubes to said carriage changes the lengths of the two tubes equally by half the displacement of the carriage.

4. A ruling engine according to claim 2 in which said means coupling said evacuated tubes to said carriage comprise respective auxiliary screws geared to the means for traversing said blank carriage.

5. A ruling engine according to claim 1 in which said servo system acts upon said tool actuating mechanism to co-ordinate the phase relationship of the mechanism to the means for traversing said carriage.

6. A ruling engine according to claim 5 comprising a photo electric cell upon which the moving fringes derived from said main interferometer as said carriage is traversed fall, thereby to generate an alternating current, operates a synchronous motor controlled by said alternating current for operating said tool actuating and mechanism, the tool being reciprocated by said mechanism.

7. A ruling engine according to claim 6 comprising a first synchronous motor which is operated by the aternating current generated by the photo-electric cell, a generator driven by said first synchronous motor for generating a second alternating current at a higher frequency and a second synchronous motor which is operated by said second alternating current and actuates the tool-reciprocating mechanism.

8. A ruling engine according to claim 6 in which to detect and correct for any departures from synchronism of said synchronous motor which operates said tool actuating mechanism amounting to a whole number of cycles of its supply, and the engine also comprises means for comparing a periodic signal derived directly from the carriage feed mechanism with a signal derived from the main interferometer beam and producing a difference signal from the comparison, a servo motor to which the difference signal is applied, a differential gear having three members to two of which said synchronous motor and said tool actuating mechanism are respectively coupled, said servo motor imposing a correcting movement corresponding to the difference signal on the third member of said differential gear.

9. A ruling engine according to claim 1 in which said tool actuating mechanism includes parts for guiding a tool in a path in which it reciprocates across a blank carried by said carriage, the engine also comprising a secondary interferometer for measuring the position of the tool with respect to a stationary element of said main interferometer thereby to detect errors and deformation in the parts guiding the tool in its traverse across the blank, and a secondary servo-system controlled by the secondary interferometer for applying any necessary correction to the position of the tool.

10. A ruling engine according to claim 9 in which the secondary interferometer includes a pair of concave mirrors each having its centre in the face of the other and the line of centres parallel to the direction of reciprocation of the tool, a symmetrical Wallaston prism attached closely to the tool, means for directing a beam to polarised light through a central aperture in one mirror on to the Wollaston prism, the other mirror being centrally apertured to permit emergence of the beam reemitted by the prism after reflection at the mirrors, means in the path of the emergent beam which detects variations in the path difference of the parts of the beam caused by longitudinal departures of the tool and Wallaston prism from the correct path and has a direct current output substantially proportional to the departures, and means responsive to the direct current to displace the tool in a direction to reduce the error.

11. A ruling engine according to claim 10 in which the two mirrors are held in position by a pair of bars of low expansion material and the stationary element of the main interferometer is polished and silvered area on one of the bars.

References Cited UNITED STATES PATENTS 2,806,293 9/1957 James et al 88-14 3,040,620 6/1962 Ferris 88l4 FOREIGN PATENTS 773,238 4/1957 Great Britain. 826,330 1/1960 Great Britain.

JEWELL H. PEDERSEN, Primary Examiner.

B. I. LACOMIS, Assistant Examiner. 

